Thought experiment: how many arcane spellcasters in a fantasy combined arms army

[Bucky]

Sorry I don't think you have given nearly enough information about the assumptions you are making for your hunter-killer squad, namely what ECL we are talking about, how many members are in the squad, and how much wealth we have to build said squad. Without any of that information your arguments are pretty moot.
For example for a group lower level than ECL 8 Floating Disk is not an effective use of your spell resources and therefore wouldn't be used most of the time. Furthermore, below ECL 8 the casters' role whether arcane or divine would optimally focus on buffing and bfc so outside of steps 3 and 4 mundane resources can be leveraged better than caster resources.

The assumptions vary based on whether I'm dictating the army's official policy or GMing a combat encounter. Here are the basic ones.

The army's assumed to have only limited access to each class - it can't simply declare that every HK squad is 5 druids and a wizard without running out of druids or severely limiting how many squads it can deploy. This limit is externally imposed - the general simply receives an army's worth of soldiers, with varying classes and levels but mostly single-classed, based on someone else's recruiting success. That said, the general considers the HK squads to be a fairly high priority for its overall spell slot expenditure on days when pitched battles seem unlikely. Also, some build choices like feats and trained skills can be standardized across a specialist subset of those holding a given class.

HK squads are each assigned a wide area to hunt in, at least two miles across. A single squad is expected to spend several days in the field at a time.

Adventuring-party-like detachments (Parties) are assumed to be in common use by the other side for various reasons. The HK squads can do some of the things that Parties do. But they specialize in finding and crippling or killing enemy Parties as well as things like messengers or lone scouts. The squads must also alert the main army to very large enemy movements.

An HK squad that thinks it's outmatched can call for reinforcements before the fight, so long as it has the actual ability to both communicate and delay the fight. A large target (10+ enemies in a group) might have several HK squads converge on it and attack at once. However, the enemy is also assumed to be able to call for help.

Soldiers first qualify for this relatively elite duty at level 4, dictated by the utility of Ranger class features. Soldiers trained for HK squads keep their roles as they level up. So HK squads vary in level from 4 to about 8 or 9, after which intensive use of divinations changes the game entirely.
- As general, this often involves using higher level mundanes to mentor minimum-level casters, and vice versa. But it also means there are strong and weak HK squads in the field at the same time.
- As GM, this means tailoring the HK squad encounter to the desired CR. It's more convenient to have everyone be the same level, but it's more challenge-efficient to tune the level of each squad member. Laziness means I'd reuse the same statblocks across several encounters - an elite member of a low level HK squad eventually becomes a regular member of a higher-level one.

These are the overall constraints I'm imposing. Tactics are downstream of these assumptions, subject to the general's judgment. For example, as designed HK squads typically have 6 members to slightly outnumber a typical Party. This is a practical limit based on the casters' limited buffs, but changing the tactics might allow larger squads.